3-54
Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements
Date Code 20001006
SEL-351 Instruction Manual
Overfrequency Element
For example, make settings:
NFREQ = 60 Hz
(nominal system frequency is 60 Hz)
E81
≥
1
(enable frequency element 1)
81D1P = 61.25 Hz.
(frequency element 1 pickup)
With these settings:
81D1P
≥
NFREQ
the overfrequency part of frequency element 1 logic is enabled. 81D1 and 81D1T operate as
overfrequency elements. 81D1 is used in testing only.
Underfrequency Element
For example, make settings:
NFREQ = 60 Hz
(nominal system frequency is 60 Hz)
E81
≥
2
(enable frequency element 2)
81D2P = 59.65 Hz
(frequency element 2 pickup)
With these settings:
81D2P < NFREQ
the underfrequency part of frequency element 2 logic is enabled. 81D2 and 81D2T operate as
underfrequency elements. 81D2 is used in testing only.
Frequency Element Operation
Refer to Figure 3.30.
Overfrequency Element Operation
With the previous overfrequency element example settings, if system frequency is less than or
equal to 61.25 Hz (81D1P = 61.25 Hz), frequency element 1 outputs:
81D1
= logical 0
(instantaneous element)
81D1T = logical 0
(time delayed element)
If system frequency is greater than 61.25 Hz (81D1P = 61.25 Hz), frequency element 1 outputs:
81D1
= logical 1
(instantaneous element)
81D1T = logical 1
(time delayed element)
Relay Word bit 81D1T asserts to logical 1 only after time delay 81D1D.
Summary of Contents for SEL-351
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